In general, a compression-type refrigerator includes at least a compressor, a condenser, an expansion mechanism (such as an expansion valve), an evaporator, and an optional drying apparatus, and a liquid mixture of refrigerant and lubricating oil (refrigerator oil) circulates in a closed system of the refrigerator. In such a refrigerator, generally, the compressor is operated at high temperature, while the cooler is operated at low temperature, although the difference in temperature varies depending on the type of the refrigerator. Therefore, a refrigerant and a lubricating oil must circulate in the system without causing phase separation within a wide temperature range of high to low temperature. Generally, a mixture of refrigerant and lubricating oil has a phase separating region in a low-temperature range and in a high-temperature range. The highest phase separation temperature in the low-temperature range is preferably −10° C. or lower, particularly preferably −20° C. or lower, whereas the lowest phase separation temperature in the high-temperature range is preferably 30° C. or higher, particularly preferably 40° C. or higher. When phase separation occurs during operation of a refrigerator, the service life and efficiency of the refrigerator are considerably impaired. For example, when phase separation between refrigerant and lubricating oil occurs in a compressor, lubrication of a movable member is impaired, thereby causing seizure or a similar phenomenon, resulting in considerable shortening of the service life of the refrigerator. When phase separation occurs in an evaporator, a viscous lubricant remains, thereby lowering heat exchange efficiency.
A chlorofluorocarbon (CFC), a hydrochlorofluorocarbon (HCFC), or the like has been heretofore mainly used as a refrigerant for a refrigerator. However, such compounds each contain chlorine that is responsible for environmental issues, so investigation has been conducted on a chlorine-free alternative refrigerant such as a hydrofluorocarbon (HFC). A hydrofluorocarbon typified by, for example, 1,1,1,2-tetrafluoroethane, difluoromethane, pentafluoroethane, or 1,1,1-trifluoroethane (hereinafter referred to as “R134a”, “R32”, “R125”, or “R143a”, respectively) has been attracting attention, and, for example, R134a has been used in a car air conditioner system.
However, because the influence of the HFC is also concerned from the viewpoint of the global warming, so-called natural refrigerants such as carbon dioxide have attracted attention as alternative refrigerants suitable for environmental protection. The carbon dioxide requires high pressure, and hence cannot be used in the current car air conditioner system.
A refrigerant having a specific polar structure in the molecules such as an unsaturated fluorinated hydrocarbon compound (see, for example, Patent Document 1), a fluorinated ether compound (see, for example, Patent Document 2), a fluorinated alcohol compound, or a fluorinated ketone compound has been found to be a refrigerant which has a low global warming potential and can be used in a current car air conditioner system.
The lubricating oil for a refrigerator that uses the refrigerant is demanded to have excellent compatibility with the refrigerant and stability.    Patent Document 1: JP 2006-503961 A    Patent Document 2: JP 07-507342 A